Field of the Invention
The invention relates to a damping arrangement for dissipating oscillating energy of an element in a system, more particularly in a microlithographic projection exposure apparatus.
Prior Art
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (i.e. for electromagnetic radiation having a wavelength of less than 15 nm), for lack of availability of light-transmissive materials, mirrors are used as optical components for the imaging process.
In the operation of projection exposure apparatuses, more particularly in the case of EUV systems, aspects relating to dynamics are becoming increasingly important for the optical performance of the system. Mechanical disturbances caused by vibrations always have a disadvantageous effect on the positional stability of the optical components. Weakly damped mechanical resonances in the system lead, in the range of resonant frequencies, to a local excessive increase in the disturbance spectrum and to an associated impairment of the positional stability of passively mounted components and also of actively controlled components. Furthermore, resonances in the case of controlled systems can lead to instability of the control loop.
Possible measures for eliminating the control instability caused by mechanical resonances, such as, for instance, reducing the control bandwidth or introducing local suppression filters (so-called “notch filter”), depending on the situation, have a disadvantageous to dramatic effect on the control performance and on the associated positional stability of the controlled optical element. In the worst case, the system can no longer be controlled stably at all. An additional aggravating factor is that the natural frequency spectra of the mechanical structures, for the dimensions of the mirrors and also of the carrying and measurement structures, the dimensions increasing as numerical apertures increase, are shifting toward lower frequencies to an ever greater extent. Consequently, oscillations that occur lead to growing problems with regard to the performance of the system and also to the effect that active position control as described initially can no longer be operated stably.
Since the (e.g. metallic or ceramic) materials permitted in EUV systems with regard to the required vacuum resistance themselves have only little intrinsic damping, further damping measures are required in order to overcome or alleviate the problems mentioned above. Various damping concepts are known in the prior art. In this respect, reference is made, for example, to WO 2006/084657 A1, WO 2007/006577 A1, DE 10 2008 041 310 A1, DE 10 2009 005 954 A1 and U.S. Pat. No. 4,123,675.
FIGS. 8a-b schematically illustrate typical conventional approaches.
In accordance with FIG. 8a, a mass-spring system comprising a mass 10 elastically suspended relative to a structure 5 via a spring 11 is damped via a damping element 12 connected in parallel with the spring 11 between mass 10 and structure 5. With regard to the required vacuum resistance, however, the choice of suitable materials for the damping element with sufficient intrinsic damping is limited in an EUV projection exposure apparatus. Further problems can result from the fact that the stiffness properties and also the damping properties of the materials can change over time.
In accordance with FIG. 8b, damping is effected via an absorber mass 15 elastically linked to the mass 10 via a further spring 13 and a damping element 14 connected in parallel therewith wherein the resonant frequencies of the respective mass-spring systems composed of absorber mass 15 and spring 13, on the one hand, and composed of mass 10 and spring 11, on the other hand, are tuned to one another in order to achieve an effective damping.
Even though the damping via an absorber mass is advantageous particularly in situations in which e.g. mechanical linking to a reference system as in FIG. 8a via dampers is not possible, with available materials such as fluoroelastomers, for example, the problem can still occur that these have, in addition to the desired damping effect, an undesirably high parasitic intrinsic stiffness which, moreover, can vary temporally over the lifetime of the system or else when there are changes in the atmosphere, and so as a result, besides the desirable damping effect, changes in the resonant frequency of the damping arrangement can also arise with the consequence that the abovementioned required tuning to the resonant frequency of the element to be damped or of the associated mass-spring system is no longer provided.